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The Averno 2 eruption (3,700±50 a B.P.) was an
explosive low-magnitude event characterized by magmatic
and phreatomagmatic explosions, generating mainly fall and
surge beds, respectively. It occurred in the Western sector of
the Campi Flegrei caldera (Campanian Region, South Italy) at
the intersection of two active fault systems, oriented NE and
NW. The morphologically complex crater area, largely filled
by the Averno lake, resulted from vent activation and
migration along the NE-trending fault system. The eruption
generated a complex sequence of pyroclastic deposits,
including pumice fall deposits in the lower portion, and
prevailing surge beds in the intermediate-upper portion. The
pyroclastic sequence has been studied through stratigraphical,
morphostructural and petrological investigations, and subdivided
into three members named A through C. Member A
was emplaced during the first phase of the eruption mainly by
magmatic explosions which generated columns reaching a
maximum height of 10 km. During this phase the eruption
reached its climax with a mass discharge rate of 3.2 106 kg/s.
Intense fracturing and fault activation favored entry of a
significant amount of water into the system, which produced
explosions driven by variably efficient water-magma interaction.
These explosions generated wet to dry surge deposits
that emplaced Member B and C, respectively. Isopachs and
isopleths maps, as well as areal distribution of ballistic
fragments and facies variation of surge deposits allow
definition of four vents that opened along a NE oriented,
2 km long fissure. The total volume of magma extruded
during the eruption has been estimated at about 0.07 km3
(DRE). The erupted products range in composition from
initial, weakly peralkaline alkali-trachyte, to last-emplaced
alkali-trachyte. Isotopic data and modeling suggest that
mixing occurred during the Averno 2 eruption between a
more evolved, less radiogenic stored magma, and a less evolved, more radiogenic magma that entered the shallow
reservoir to trigger the eruption. The early phases of the
eruption, during which the vent migrated from SW to the
center of the present lake, were fed by the more evolved,
uppermost magma, while the following phases extruded the
less evolved, lowermost magma. Integration of the geological
and petrological results suggests that the Averno 2 complex
eruption was fed from a dyke-shaped shallow reservoir
intruded into the NE-SW fault system bordering to the west
the La Starza resurgent block, within the caldera floor.